1. Field of the Invention
This invention relates to composite oxide powders useful as a catalyst support, a method of producing the same and a catalyst using this composite oxide powder as a catalyst support. This catalyst can be utilized for the purposes of hydrogen generation, exhaust gases purification, and so on.
2. Description of the Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NOx) at the same time to purify exhaust gases. For example, 3-way catalysts are known widely which comprise a heat-resistant honeycomb-shaped supporting base material formed of cordierite, and a catalyst support layer formed of γ—Al2O3 and disposed on the supporting base material, and a noble metal such as platinum (Pt) and rhodium (Rh) loaded on the catalyst support layer.
Catalyst supports used for catalysts for purifying exhaust gases are required to have a large specific surface area and a high heat resistance, and are generally composed of Al2O3, SiO2, ZrO2, TiO2 or the like. In order to reduce variations in exhaust gases atmospheres, it is also known to add CeO2, which has oxygen storage ability, or a CeO2—ZrO2 solid solution, which has improved oxygen storage ability and heat resistance to CeO2.
By the way, owing to the recent severe regulations on exhaust gases, there is an extremely strong need to purify exhaust gases even in a very short time from an engine start. To meet these regulations, it is necessary to activate a catalyst to purify regulated exhaust gases components at lower temperatures. Particularly, a catalyst comprising CeO2 and Pt, etc., loaded thereon has a superior ability of converting CO from a low temperature. By using this type of catalysts in combination with the above 3-way catalysts, CO is ignited at low temperature and CO adsorption poisoning is lessened and HC ignitability improves. Also by igniting CO at low temperature, warming of catalyst surfaces is promoted, whereby HC can be converted from a low temperature range. Besides, when this type of catalysts are employed, H2 is generated in a low temperature range by water gas shift reaction and the generated H2 can be used as a reducing agent for a reaction with NOx, whereby NOx can be reduced and purified from a low temperature range.
However, the conventional catalyst comprising CeO2 and Pt or the like loaded on CeO2 is poor in durability in actual exhaust gases because CeO2 is sintered by heat, and cannot be in practical use. In order to use this conventional catalyst in actual exhaust gases, there is need to improve its heat resistance without damaging oxygen storage and release capability of CeO2. If the heat resistance is not improved without damaging the oxygen storage capability of CeO2, there is a fear that noble metal grains grow and the noble metal catalytic activity declines. Therefore, when CeO2 is employed as a catalyst support, it is essential to stabilize a noble metal on the catalyst support.